The Official Seasons

Ever wonder what the weatherperson means when they say that such-and-such day is the "official" start of one of the four seasons? What makes it official, and who decided?

The answer is that the traditional boundaries between the four seasons are related to the motion of the Sun, and the tradition dates from antiquity. Throughout the course of a year, the Sun's position on the horizon as it rises and sets each day slowly shifts north and south. This is simple geometry. The axis of the Earth's spin is canted at a 23° angle with respect to the plane of the Earth's orbit around the Sun, which therefore means that the Sun's position in the sky depends very much on where the Earth is in its orbit.

Consider this schematic of the Earth's orbit. Note that the Earth's axis always points in the same direction: it does not rotate or shift as the Earth orbits the Sun. When the Earth is at the point in its orbit where its axis is pointing at the Sun (far left), then people in the Northern Hemisphere are exposed to the Sun for the maximum amount of time. We have summer. Likewise, when the Earth is on the opposite side of its orbit, with its axis pointing away from the Sun, then people in the Northern Hemisphere are exposed to the Sun for the minimum amount of time, and we have winter.

Challenge - Can you deduce why it is that the seasons in the Southern Hemisphere are exactly opposite to those in the Northern?

Halfway between these points (top and bottom of figure), the Sun is positioned directly perpendicular to the Earth's axis, which means all of the Earth is in sunlight for 12 hours, and in darkness for 12 hours.

From the viewpoint of someone on the Earth, this means that the rising/setting point of the Sun appears to move along the horizon during the course of a year, which makes its daily path through the sky longer or shorter. The longest day of the year, when the Sun rises at its northernmost point, is called the Summer Solstice. The shortest day of the year, when the Sun rises at its southernmost point, is called the Winter Solstice. The two days exactly in between the solstices, when the amount of daylight and darkness are equal, are known as the equinoxes.

So, these points in the Sun's motion define the seasons:

  • Summer Solstice: official start of summer (about June 21, left of figure)
  • Autumnal Equinox: official start of fall (about September 21, bottom of figure)
  • Winter Solstice: official start of winter (about December 21, right of figure)
  • Vernal Equinox: official start of spring (about March 21, top of figure)
I say "about" for the dates because the Earth's orbit and the calendar we use are not exactly synchronized (if they were, we wouldn't need leap years). So, the exact dates can shift by a day, even though the 21st of each month is usually correct.

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